1. Field of the Invention
This invention is related in general to the field of temperature and flow controllers and, in particular, to methods and apparatus suitable for digital operation.
2. Description of the Related Art
Everyone is familiar with the process of adjusting the water temperature out of a faucett or shower head by balancing the flow rate through hot and cold water valves. Obviously, in order to raise the outlet water temperature, either the hot water flow is increased or the cold water flow is decreased; and the opposite procedure is followed to lower the water temperature. This process necessarily affects the overall flow rate and often requires a readjustment of each valve to produce the desired total output as well as the desired temperature.
Thus, the process may lead to an undesirable waste of water and time, hardly acceptable in a modern world of heightened ecological consciousness, comfort and computerized appliances. Therefore, several devices have been developed to control both the flow rate and the temperature of water flowing to an outlet. In particular, U.S. Pat. No. 4,170,245 to Haley (1979) describes a digital control valve comprising a plurality of parallel channels containing separate on/off valves actuated by individual solenoids. Each channel is sized in binary progression, so as to deliver flow rates compatible with computer logic for digital control. The overall flow rate can be adjusted to a desired quantity by selecting the appropriate channels of flow; the rate will be precise within a tolerance determined by the smallest orifice.
U.S. Pat. No. 4,757,943 to Sperling et. al. (1988) uses the same concept in the design of a liquid-flow temperature controller. By combining a valve of the type described by Haley with a temperature sensor and digital controller, this invention provides an apparatus for automatically setting the liquid temperature to a desired level. The flow of both hot and cold water is regulated by separate flow controllers consisting of a plurality of digital, solenoid-driven on/off valves; the temperature of the resulting mixture is measured by a sensor and fed to a temperature controller; the controller determines an error with respect to a setpoint and actuates the appropriate solenoid/driven valves according to a predetermined feedback control scheme. This patent also takes into account the nonlinearity between flow rates and orifice diameters, and the effect of temperature on viscosity, in order to produce a precise balancing of flow rates to achieve a targeted temperature.
These systems utilize banks of on/off solenoid valves that are either completely open, thus allowing a flow proportional to the size of the orifice in the valve, or completely closed, thus allowing no flow at all. In regulating flow rate, the correct combination of valves to produce the desired rate is kept open, leaving the balance closed. This permits a binary system of control for achieving a precise flow rate within the capacity of the smallest valve. Similarly, in regulating temperature, the correct number of valves on the hot and cold water banks are opened to produce respective flow rates that yield the desired combined temperature. As each valve is either opened or closed, though, the total flow rate is affected, thus requiring the same process of temperature and flow rate balancing inherent with manual operation.
Even if the two inventions are combined for automatic flow and temperature control, the two units will necessarily affect one another and function dependently, so that the control of the two parameters will require an iterative control procedure converging to the desired setpoints. This is essentially what occurs when temperature and flow rate are set manually. Therefore, there still exists a need for a valve system that automatically and independently controls the rate of delivery and the temperature of a liquid to desired setpoints.